diff --git a/coderd/database/dump.sql b/coderd/database/dump.sql
index 327a98638a2ad..fb7ab0b8e4079 100644
--- a/coderd/database/dump.sql
+++ b/coderd/database/dump.sql
@@ -4770,7 +4770,7 @@ CREATE INDEX idx_chat_messages_chat_created ON chat_messages USING btree (chat_i
 
 CREATE INDEX idx_chat_messages_chat_role_id ON chat_messages USING btree (chat_id, role, id DESC) WHERE (deleted = false);
 
-CREATE INDEX idx_chat_messages_compressed_summary_boundary ON chat_messages USING btree (chat_id, created_at DESC, id DESC) WHERE ((compressed = true) AND (role = 'system'::chat_message_role) AND (visibility = ANY (ARRAY['model'::chat_message_visibility, 'both'::chat_message_visibility])));
+CREATE INDEX idx_chat_messages_compressed_summary_boundary ON chat_messages USING btree (chat_id, id DESC) WHERE ((compressed = true) AND (deleted = false) AND (visibility = 'model'::chat_message_visibility));
 
 CREATE INDEX idx_chat_messages_created_at ON chat_messages USING btree (created_at);
 
diff --git a/coderd/database/migrations/000560_chat_summary_boundary_index_by_id.down.sql b/coderd/database/migrations/000560_chat_summary_boundary_index_by_id.down.sql
new file mode 100644
index 0000000000000..f0bb3a2f3c9ff
--- /dev/null
+++ b/coderd/database/migrations/000560_chat_summary_boundary_index_by_id.down.sql
@@ -0,0 +1,7 @@
+DROP INDEX idx_chat_messages_compressed_summary_boundary;
+
+CREATE INDEX idx_chat_messages_compressed_summary_boundary
+	ON chat_messages(chat_id, created_at DESC, id DESC)
+	WHERE compressed = TRUE
+		AND role = 'system'
+		AND visibility IN ('model', 'both');
diff --git a/coderd/database/migrations/000560_chat_summary_boundary_index_by_id.up.sql b/coderd/database/migrations/000560_chat_summary_boundary_index_by_id.up.sql
new file mode 100644
index 0000000000000..914bea3397ce1
--- /dev/null
+++ b/coderd/database/migrations/000560_chat_summary_boundary_index_by_id.up.sql
@@ -0,0 +1,10 @@
+-- The predicate required role = 'system', but compaction writes its summary
+-- with the user role, so this index has never matched a row. Rebuild it to
+-- match GetChatMessagesForPromptByChatID's boundary lookup, which orders by id.
+DROP INDEX idx_chat_messages_compressed_summary_boundary;
+
+CREATE INDEX idx_chat_messages_compressed_summary_boundary
+	ON chat_messages(chat_id, id DESC)
+	WHERE compressed = TRUE
+		AND deleted = false
+		AND visibility = 'model';
diff --git a/coderd/database/querier.go b/coderd/database/querier.go
index 0eb2f1d78d924..eefcf9eadcf36 100644
--- a/coderd/database/querier.go
+++ b/coderd/database/querier.go
@@ -469,6 +469,8 @@ type sqlcQuerier interface {
 	GetChatMessagesByChatIDDescPaginated(ctx context.Context, arg GetChatMessagesByChatIDDescPaginatedParams) ([]ChatMessage, error)
 	// Stream deltas and reset snapshots must use the same message order.
 	GetChatMessagesByRevisionForStream(ctx context.Context, arg GetChatMessagesByRevisionForStreamParams) ([]ChatMessage, error)
+	// The compaction boundary and final ordering must use the same key so tool
+	// results remain after their assistant calls.
 	GetChatMessagesForPromptByChatID(ctx context.Context, chatID uuid.UUID) ([]ChatMessage, error)
 	GetChatModelConfigByID(ctx context.Context, id uuid.UUID) (ChatModelConfig, error)
 	GetChatModelConfigs(ctx context.Context) ([]ChatModelConfig, error)
diff --git a/coderd/database/querier_test.go b/coderd/database/querier_test.go
index c5511ebda3c1f..46a25dd10434d 100644
--- a/coderd/database/querier_test.go
+++ b/coderd/database/querier_test.go
@@ -12346,7 +12346,7 @@ func TestGetChatMessagesForPromptByChatID(t *testing.T) {
 
 	// This test exercises a complex CTE query for prompt
 	// reconstruction after compaction. It requires Postgres.
-	db, _ := dbtestutil.NewDB(t)
+	db, _, sqlDB := dbtestutil.NewDBWithSQLDB(t)
 	ctx := context.Background()
 
 	// Helper: create a chat model config (required FK for chats).
@@ -12426,14 +12426,49 @@ func TestGetChatMessagesForPromptByChatID(t *testing.T) {
 		return database.ChatMessage(results[0])
 	}
 
-	msgIDs := func(msgs []database.ChatMessage) []int64 {
-		ids := make([]int64, len(msgs))
-		for i, m := range msgs {
-			ids[i] = m.ID
-		}
-		return ids
+	invertCreatedAt := func(t *testing.T, chatID uuid.UUID) {
+		t.Helper()
+		_, err := sqlDB.ExecContext(ctx,
+			"UPDATE chat_messages SET created_at = now() - (id || ' seconds')::interval WHERE chat_id = $1",
+			chatID)
+		require.NoError(t, err)
 	}
 
+	t.Run("OrdersByIDWhenTimestampsDisagree", func(t *testing.T) {
+		t.Parallel()
+		chat := newChat(t)
+
+		sys := insertMsg(t, chat.ID, database.ChatMessageRoleSystem, database.ChatMessageVisibilityModel, false, "system prompt")
+		usr := insertMsg(t, chat.ID, database.ChatMessageRoleUser, database.ChatMessageVisibilityBoth, false, "question")
+		ast := insertMsg(t, chat.ID, database.ChatMessageRoleAssistant, database.ChatMessageVisibilityBoth, false, "tool call")
+		tool := insertMsg(t, chat.ID, database.ChatMessageRoleTool, database.ChatMessageVisibilityBoth, false, "tool result")
+		invertCreatedAt(t, chat.ID)
+
+		got, err := db.GetChatMessagesForPromptByChatID(ctx, chat.ID)
+		require.NoError(t, err)
+		require.Equal(t, []int64{sys.ID, usr.ID, ast.ID, tool.ID}, chatMessageIDs(got),
+			"the prompt must keep append order so a tool result follows its assistant call")
+	})
+
+	t.Run("CompactionBoundaryUsesID", func(t *testing.T) {
+		t.Parallel()
+		chat := newChat(t)
+
+		sys := insertMsg(t, chat.ID, database.ChatMessageRoleSystem, database.ChatMessageVisibilityModel, false, "system prompt")
+		insertMsg(t, chat.ID, database.ChatMessageRoleUser, database.ChatMessageVisibilityBoth, false, "before first summary")
+		staleSummary := insertMsg(t, chat.ID, database.ChatMessageRoleUser, database.ChatMessageVisibilityModel, true, "first summary")
+		insertMsg(t, chat.ID, database.ChatMessageRoleUser, database.ChatMessageVisibilityBoth, false, "between summaries")
+		latestSummary := insertMsg(t, chat.ID, database.ChatMessageRoleUser, database.ChatMessageVisibilityModel, true, "second summary")
+		afterLatest := insertMsg(t, chat.ID, database.ChatMessageRoleUser, database.ChatMessageVisibilityBoth, false, "after second summary")
+		invertCreatedAt(t, chat.ID)
+
+		got, err := db.GetChatMessagesForPromptByChatID(ctx, chat.ID)
+		require.NoError(t, err)
+		require.Equal(t, []int64{sys.ID, latestSummary.ID, afterLatest.ID}, chatMessageIDs(got),
+			"the boundary is compared with id, so it must also be selected by id")
+		require.NotContains(t, chatMessageIDs(got), staleSummary.ID)
+	})
+
 	t.Run("NoCompaction", func(t *testing.T) {
 		t.Parallel()
 		chat := newChat(t)
@@ -12444,7 +12479,7 @@ func TestGetChatMessagesForPromptByChatID(t *testing.T) {
 
 		got, err := db.GetChatMessagesForPromptByChatID(ctx, chat.ID)
 		require.NoError(t, err)
-		require.Equal(t, []int64{sys.ID, usr.ID, ast.ID}, msgIDs(got))
+		require.Equal(t, []int64{sys.ID, usr.ID, ast.ID}, chatMessageIDs(got))
 	})
 
 	t.Run("UserOnlyVisibilityExcluded", func(t *testing.T) {
@@ -12463,7 +12498,7 @@ func TestGetChatMessagesForPromptByChatID(t *testing.T) {
 			require.NotEqual(t, database.ChatMessageVisibilityUser, m.Visibility,
 				"visibility=user messages should not appear in the prompt")
 		}
-		require.Contains(t, msgIDs(got), usr.ID)
+		require.Contains(t, chatMessageIDs(got), usr.ID)
 	})
 
 	t.Run("AfterCompaction", func(t *testing.T) {
@@ -12490,7 +12525,7 @@ func TestGetChatMessagesForPromptByChatID(t *testing.T) {
 		got, err := db.GetChatMessagesForPromptByChatID(ctx, chat.ID)
 		require.NoError(t, err)
 
-		gotIDs := msgIDs(got)
+		gotIDs := chatMessageIDs(got)
 
 		// Must include: system prompt, summary, post-compaction.
 		require.Contains(t, gotIDs, sys.ID, "system prompt must be included")
@@ -12529,8 +12564,8 @@ func TestGetChatMessagesForPromptByChatID(t *testing.T) {
 		}
 		require.True(t, hasNonSystem,
 			"prompt must contain at least one non-system message after compaction")
-		require.Contains(t, msgIDs(got), summary.ID)
-		require.Contains(t, msgIDs(got), newUsr.ID)
+		require.Contains(t, chatMessageIDs(got), summary.ID)
+		require.Contains(t, chatMessageIDs(got), newUsr.ID)
 	})
 
 	t.Run("CompressedToolResultNotPickedAsSummary", func(t *testing.T) {
@@ -12549,7 +12584,7 @@ func TestGetChatMessagesForPromptByChatID(t *testing.T) {
 		got, err := db.GetChatMessagesForPromptByChatID(ctx, chat.ID)
 		require.NoError(t, err)
 
-		gotIDs := msgIDs(got)
+		gotIDs := chatMessageIDs(got)
 		require.Contains(t, gotIDs, summary.ID, "real summary must be included")
 		require.NotContains(t, gotIDs, compressedTool.ID,
 			"compressed tool result must not be included")
diff --git a/coderd/database/queries.sql.go b/coderd/database/queries.sql.go
index 30bf07a624ae6..97c31e6b891be 100644
--- a/coderd/database/queries.sql.go
+++ b/coderd/database/queries.sql.go
@@ -8417,7 +8417,6 @@ WITH latest_compressed_summary AS (
         AND deleted = false
         AND visibility = 'model'
     ORDER BY
-        created_at DESC,
         id DESC
     LIMIT
         1
@@ -8460,10 +8459,11 @@ WHERE
         )
     )
 ORDER BY
-    created_at ASC,
     id ASC
 `
 
+// The compaction boundary and final ordering must use the same key so tool
+// results remain after their assistant calls.
 func (q *sqlQuerier) GetChatMessagesForPromptByChatID(ctx context.Context, chatID uuid.UUID) ([]ChatMessage, error) {
 	rows, err := q.db.QueryContext(ctx, getChatMessagesForPromptByChatID, chatID)
 	if err != nil {
diff --git a/coderd/database/queries/chats.sql b/coderd/database/queries/chats.sql
index fec4a006aecbc..b672ec5068a1b 100644
--- a/coderd/database/queries/chats.sql
+++ b/coderd/database/queries/chats.sql
@@ -484,6 +484,8 @@ LIMIT
     COALESCE(NULLIF(@limit_val::int, 0), 500);
 
 -- name: GetChatMessagesForPromptByChatID :many
+-- The compaction boundary and final ordering must use the same key so tool
+-- results remain after their assistant calls.
 WITH latest_compressed_summary AS (
     SELECT
         id
@@ -495,7 +497,6 @@ WITH latest_compressed_summary AS (
         AND deleted = false
         AND visibility = 'model'
     ORDER BY
-        created_at DESC,
         id DESC
     LIMIT
         1
@@ -538,7 +539,6 @@ WHERE
         )
     )
 ORDER BY
-    created_at ASC,
     id ASC;
 
 -- name: GetChats :many